1. Field of the Invention
The present invention relates to a signal processing apparatus for detecting a flicker from an image signal obtained from a CMOS (Complementary Metal Oxide Semiconductor) type image capturing device and the like, which performs a photoelectric conversion on light from a subject and captures an image of the subject; a signal processing method; a control program including processing steps for allowing a computer to execute each step of the signal processing method; a computer-readable recording medium storing the control program as data; a solid-state image capturing apparatus having the signal processing apparatus used therein; and an electronic information device, such as a digital camera (e.g., a digital video camera and a digital still camera), an image input camera, a scanner, a facsimile machine, and a camera-equipped cell phone device, having the solid-state image capturing apparatus with the signal processing apparatus as an image input device used in an image capturing section thereof.
2. Description of the Related Art
Under a fluorescent light (lamp) using an alternating current source except for the inverter type, lateral striped light and dark portions, referred to as flicker noise, conventionally appear on a photographed image of a display screen when the image is taken by a CMOS type image capturing device. This is due to the change in the amount of the fluorescent light in accordance with cycles of the power source voltage.
In regard to this problem, Reference 1 proposes a method for detecting a flicker frequency from an image signal that is processed to obtain a photographed image in a pre-processing section.
FIG. 14 is a block diagram illustrating an exemplary essential configuration of a conventional digital signal processing apparatus disclosed in Reference 1.
In FIG. 14, the conventional digital signal processing apparatus is used for an image capturing apparatus, and includes a pre-processing section 50; a flicker detecting and reducing section 60; and a post-processing section 70.
The pre-processing section 50 includes a digital clamp circuit 51; an inter-read-out channel gain correction circuit 52; a fixed pattern noise reduction circuit 53; a defective pixel data correction circuit 54; a noise reduction circuit 55; a lens shading correction circuit 56; and a digital gain adjustment circuit 57. That is, the pre-processing section 50 clamps an RGB primary color signals, which are A/D converted after being obtained from an image capturing element; corrects the gain between read-out channels with regard to the RGB primary color signals having clamped black levels; reduces fixed pattern noise; corrects data of defective pixels; reduces noise; corrects lens shading; and adjusts the gain for exposure adjustment.
When signals inputted from an analog signal processing section are RGB primary color signals, the digital clamp circuit 51 clamps the black level of the inputted RGB primary color signals at a predetermined level.
The inter-read-out channel gain correction circuit 52 corrects a signal gain for the RGB primary color signals with the clamped black level of each read-out channel, in such a manner to eliminate the variation of the signal gain for each read-out channel, when image-capturing signals are read out from a CMOS image capturing device by a plurality of read-out channels.
The fixed pattern noise reduction circuit 53 reduces vertical striped fixed pattern noise for the RGB primary color signals after the inter-read-out channel gain correction.
The defective pixel data correction circuit 54 corrects data (pixel value) of a defective pixel detected by a defective pixel detection circuit 74 in a post-processing section 70, in the RGB primary colors after the fixed noise pattern reduction. Particularly, data of a defective pixel is calculated and generated by an interpolation operation from data of pixels around the defective pixel, and the original data is replaced with the calculated and generated data.
The noise reduction circuit 55 reduces noise for the RGB primary colors after the defective pixel data correction. The noise in this case is a general high frequency noise different from the fixed pattern noise described above. Particularly, a filtering process is performed to extract only a low frequency component from the RGB primary color signals to reduce the noise.
The lens shading correction circuit 56 corrects lens shading (which is a phenomenon where signal amounts differ from each other depending on image focusing locations, such at a middle part or a peripheral part, and in general, an image becomes darker and the signal amount decreases as the location moves further away from the center of the optical axis of a lens) for the RGB primary color signals after the noise reduction.
The digital gain adjustment circuit 57 performs brightness adjustment by adjusting a gain of the RGB primary color signals after the lens shading correction for exposure adjustment, by the gain setting of a system controller (not shown). In the image capturing apparatus, the exposure amount can be adjusted by the iris setting, the electronic shutter speed setting, and the gain adjustment by the digital gain adjustment circuit 57. By adjusting the gain by the digital gain adjustment circuit 57 operating together with the iris setting and the electronic shutter speed setting, a desired exposure amount can be obtained and an automatic exposure adjustment function can be achieved as well.
The flicker detecting and reducing section 60 detects a flicker of a fluorescent light and reduces a flicker component.
The post-processing section 70 includes a white balance adjustment circuit 71; a gamma correction circuit 72; a matrix synthesizing circuit 73; and a defective pixel detection circuit 74. The white balance is adjusted for the RGB primary color signals after the flicker reduction, a defective pixel of the image capturing device is detected from the RGB primary color signals after the white balance adjustment, and a gamma correction is performed, and a brightness signal Y and color difference signals R-Y and B-Y are generated.
The white balance adjustment circuit 71 adjusts each gain for the white balance that normalizes the balance of the signal level of each of the RGB primary signals.
The gamma correction circuit 72 changes the gradation of the RGB primary color signals after the white balance adjustment in accordance with the gradation characteristic of a display apparatus.
The matrix synthesizing circuit 73 generates a brightness signal Y, color difference signals R-Y and B-Y, as the output of a digital signal processing apparatus, from the RGB primary color signals after the gamma correction.
The defective pixel detection circuit 74 detects a defective pixel of the CMOS image capturing device from the output signal from the white balance adjustment circuit 71, and outputs the defective pixel detection result to the defective pixel data correction circuit 54.    Reference 1: Japanese Laid-Open Publication No. 2005-347939